Polymer electrolyte membrane fuel cell (PEMFC) systems electrochemically react a hydrogen fuel with an oxidant to produce electricity, with the by-product being only heat and water when pure hydrogen is used as the fuel.
It is known to use metal or metal oxide particles as catalysts in fuel cell applications. In a PEMFC it is common to use platinum-based catalysts including carbon supported platinum, and carbon supported platinum alloys with palladium and other metals. Platinum catalysts provide excellent hydrogen electrochemical activity and good durability in a strong acidic media such as polytetrafluoroethylene (PTFE) resin particles, of which the membrane is usually made.
The high cost and price volatility of platinum makes it desirable to minimize its usage in fuel cells. Attempts have been made to form thinner films of platinum on carbon support on the electrodes. By this method it has been possible to reduce the usage of platinum-based particles from about 8 mg/cm2 in 2005 down to about 0.3 mg/cm2 in 2010. In research settings loads of platinum as low as 0.15 mg/cm2 have been achieved on the anode side. However, the loading of platinum on the cathode side is still high, which increases the cost of PEMFC systems.
It is desirable that a catalyst for use in a fuel cell system demonstrate good catalytic activity and durability. Significant electrochemical properties of a catalyst include the specific surface area (active surface area), the structure, the composition, and catalytic activity. Reducing the size of platinum particles below about 4 nanometers showed a reduction in total electrochemical activity even though the smaller size can increase the total surface area. Platinum nanoparticles of around 4 nm or higher are thus considered desirable for use in PEMFC systems.
Typically, the platinum nanoparticles used in PEMFC systems have a spherical or distorted spherical shape. A portion of the particle is not available for catalysis because it is attached to the substrate. Further, certain exposed surfaces of the nanoparticles will not be well utilized because large molecules such as oxygen have a lower probability of accessing the active sites on the surface of spherical nanoparticles when compared to smaller molecules like hydrogen.
In addition, for spherical particles, because most catalytic reactions are surface reactions, the inner part of the spherical particles that consists of the most weight is not utilized at all. Therefore, spherical shape particles for catalyst reaction are not ideal.
A catalytic reaction depends on the large surface areas of the catalyst, the catalytic activity of the catalyst, and reaction conditions. The active sites of the catalysts are particularly important and associated directly to the catalytic activity. It is well documented that more grain boundaries, crystal defects including twins, dislocations, mismatches, and junctions between different elements or different chemical states of the same elements promote catalytic activity of the reaction.
Manipulation of other parameters in a fuel cell system such as air pressure can improve catalytic performance but in general will not completely overcome the intrinsic disadvantages of spherical nanoparticles, because the inner part (non-surface portion) of the nanoparticles remain unutilized despite manipulation of the air pressure. In addition, it can be difficult to enhance the active site of a defined size spherical nanoparticle especially if they are optimized for the processing conditions, such as preparation of platinum nanoparticles by impregnation or thermal reduction means.
Various methods for the production of nanoparticle films are known. For example, U.S. Pat. No. 6,458,431 discloses a method for depositing nanoparticles as an amorphous thin film through a solid state film of precursors from a solution which is deposited on a substrate and converted into a metal or metal oxide film. This method can produce amorphous and some metallic thin films from a solid state film of metal organic complexes in air or under other gas conditions. The shape of the nanoparticles are mostly irregular, some of them are spherical.
US 2004/191423 discloses photoresist-free method for depositing films composed of metal and metal oxide from metal organic complexes. This method can be used to print micron or submicron sized patterns by irradiation of the metal organic complexes in a solid state film. The produced nanoparticles in amorphous form or some in metallic form are packed with pores. The nanoparticles form a thin film with a thickness range from 20 to a few hundreds nanometer.
US 2008/085326 discloses novel antimicrobial materials comprising of polycrystalline nanoparticles of metal, metal oxide, and active oxygen species in a permeable structure, which has nothing related to catalyst on nanosized supports as well.
Accordingly, it is an object of an embodiment of the present invention to provide a nanoparticle catalyst providing improved catalytic activity at a low load.
It is an object of a further embodiment of the invention to provide a method of producing such a nanoparticle catalyst.
Other objects of the invention will be apparent from the description that follows.